1. Field of the Invention
This invention relates generally to the field of analog integrated circuit design and, more particularly, to voltage reference design.
2. Description of the Related Art
In the design of most analog integrated circuits, more specifically when using complementary metal-oxide-semiconductor (CMOS) technology, a stable reference voltage is often required to insure proper circuit operation. Functional circuit elements of these integrated circuits often employ current sources that are expected to provide stable currents. Such circuit elements can be current mirrors, differential amplifiers or operational amplifiers. Because field effect transistors are commonly used to implement current sources, the stability of such current sources is greatly affected by the stability and accuracy of the reference voltage applied to the gate of the field effect transistor (FET).
An example of an integrated circuit function that requires a precise voltage reference is an analog to digital converter (ADC). Other examples may include analog filters whose gain and rolloff frequency characteristics are dependent upon their bias currents. In another example, control circuits designed to control the switching response of logic circuits often employ a series FET whose gate is controlled by a reference voltage, affecting the slew rate and/or switching speed of the control circuit. Voltage references in such control circuits are preferred to provide accurate and stable reference voltages.
Typically, voltage reference values need to be adjusted to account for possible process variations. When working within generally tight operational tolerances, a trim capability of the circuit may be required to make the necessary adjustments needed for achieving proper circuit operation over variations present in silicon processing. This trim capability is commonly implemented by way of fuses that are usually cut, or uncut, for adjusting the reference voltage. Often, the trimming is performed by probing wafers during device characterization, determining the required trim, then cutting the fuses using laser trimming-devices. This method, however, may not provide sufficient control over the voltage reference since it is used during characterization and probe, not during normal circuit operations. For example, temperature variations in the die may cause a reference voltage drift during normal operations. Since the fuses are cut during characterization and probe, they would provide no means of controlling such a drift. While it is possible to control the drift during normal circuit operations using complex analog circuits, such methods are in many cases impractical and limited by die size and other factors. A more viable solution may be to employ methods utilizing simpler, more efficient digital control functions.
Therefore, there exists a need for a system and method for dynamically and digitally controlling a voltage reference, compensating for the reference voltage drift caused by temperature variations.